Effects of supplied nitrogen form on growth and water uptake of French bean ( Phaseolus vulgaris L.) plants

Abstract

In order to investigate the effect of N form on dry matter (DM) formation and water uptake rate, French bean (Phaseolus vulgaris L. `Sotaxa') plants were grown with a split-root system. Three treatments were compared: sole nitrate (NO− 3) supply (NN), sole ammonium (NH+ 4) supply (AA) and spatially separated supply of NO− 3 and NH+ 4 (NA). The pH of the nutrient solutions was kept constant at 6.3 using a pH-stat system. 9 days after onset of the treatments, NN plants had higher root (36%) and shoot dry matter (11%) than AA plants. N form drastically influenced partitioning of assimilates: in the NA treatment, the root half exposed to NO− 3 revealed a 170% higher DM than the root half exposed to NH+ 4. N form affected stable carbon-isotope discrimination (δ) of leaf tissue. In leaves of plants which were supplied with NH+ 4 (AA; NA) δ was significantly more negative (–29.4‰, –29.6‰) than in NN treatment (–28.2‰). We explain this effect by differences in stomatal conductance. We suppose that the significantly less negative δ of root tissue under NH+ 4 supply is most probably related to higher PEP-case activity. The water uptake rate was higher in NN than in AA grown plants. This effect was found in both, short- and long-term experiments. In case of NA plants, the water uptake in the root part being exposed to NO− 3 was 104% higher than in those receiving NH+ 4. At least in the case of the NA treatment we can exclude shoot growth effects as being responsible for differences in water uptake. We therefore assume that differences in root hydraulic conductivity are responsible for the observed effects.